Cryo-EM Core Abstract The Cryo-EM Core will be an important component of the PCHPI structure determination pipeline, playing a major role in Project 1 and significantly contributing to Projects 2, 3 and 4, and tackling new HIV-host protein complexes as they become available. The Core will screen complexes produced in the Protein Core or by collaborators for their suitability for Cryo-EM or Cryo-ET structure determination and will determine the structures of HIV-host protein complexes at atomic or near-atomic resolution, using single particle methodologies. In addition, the Cryo-EM Core will develop methods to analyze structural heterogeneity within samples, an issue that complicates current EM-based analytical approaches.